The present invention relates to the field of coated hard refractory substrates, especially cutting tools.
In the past, cemented carbide cutting tools have been used in both a coated and an uncoated condition to machine metals and alloys. The application of a coating having one or more layers of a refractory material to a tungsten carbide-cobalt cemented carbide substrate has been used to improve the wear resistance and application range of cemented carbide cutting tools. In the past, refractory coatings, such as TiC, TiCN, TiN, and Al.sub.2 O.sub.3, have been applied by CVD (chemical vapor deposition) techniques. In addition, TiN coatings have been applied by PVD (physical vapor deposition) techniques. Such CVD coatings deposited on cemented carbide substrates are characterized by thermal cracks, and residual tensile stresses. PVD TiN coatings are characterized by a dense, pore free structure without thermal cracks, and may have residual compressive stresses. The application of CVD coatings to cemented carbide substrates results in a reduction in the transverse rupture strength of the insert and, therefore, produces a greater susceptibility to chipping and breakage during use.
Titanium alloys are particularly difficult to machine due to the high degree of reactivity between the cutting tool and the hot metal chips which flow across it. Chipping and breakage of the cutting edge are significant problems in this application. One of the best tool materials to use to machine titanium is an uncoated cemented carbide grade of material (see substrate 2 described subsequently). However, because it is uncoated, its application range is limited to low speeds. Some improvements have been achieved by coating this substrate with a PVD TiN coating.
Other difficult to machine materials are the high temperature superalloys (iron, nickel and/or cobalt base). While ceramics have been used to rough machine these materials, final or finish machining is done by cemented carbides at low speeds. Cemented carbides are used for finishing machining because their high toughness makes them less likely to break or chip severely during machining. Such a failure might ruin the surface finish of expensive turbine or aircraft components, leading to expensive re-working or scrapping of the almost finished component. Advances in turbine and aircraft materials have led to new alloys which allow aircraft engines to operate at higher temperatures. Such materials (e.g., PWA-1074; IN-100) are highly abrasive and are extremely difficult to machine.
There is definitely a need for a new cutting tool material for machining these difficult to machine materials.